The present invention relates to the surface treatment of high-nickel/iron alloy steel plate for LNG or LPG tanks provided by ASTM SA-658 to improve its corrosion resistance, in which the alloy steel plate is subjected to nickel plating and chromating (except electro-chromating).
Stainless steel or aluminum steel plates have mainly been used as the structural material for LNG or LPG tanks. These plates excel in corrosion resistance, and offer no particular problem in connection with rusting. However, the tanks referred to above are inevitably subject to a thermal cycle in which a temperature rise and a temperature drop occur repeatedly. In other words, the tank material does not stand up to use over extended periods, owing to its fatigue caused by repeated expansion and contraction. This is the reason why another material is now demanded. It is true that the most suitable material for LNG or LPG tanks is high-nickel/iron alloy steel plate provided by ASTM SA-658 that has an extremely low coefficient of thermal expansion (about 1.0.times.10.sup.-6 -2.0.times.10.sup.-6 /.degree.C.) in view of fatigue but, as well-known in the art, this material is deficient in corrosion resistance, viz., shows a degree of corrosion resistance no more than that of normal-purpose steel plate, thus leaving behind a rusting problem before or after making tanks.
To prevent the high-nickel/iron alloy plate for LNG or LPG tanks from rusting, there may be two possibilities; one is the application of rust preventing oil, and the other is the application of a rust preventing film.
The application of rust preventing oil poses a slip problem and is dangerous at work, viz., unsatisfactory in view of workability, whereas the application of a rust preventing film is very costly since, upon coating, the film has to be removed by timeconsuming and laborious working, especially for welding purposes. If welding is effected with the alloy plate still coated with the rust preventing oil or film, blow holes or cracks then occur in the welded portion, resulting in a drop of the strength of the welded joint.